Liquid metal manoeuvred into new applications

A technique for controlling the surface tension of liquid metals could lead to applications in reconfigurable electronic circuits, antennas and other technologies.

Developed at North Carolina State University, the technique is said to hinge on the fact that the oxide “skin” of the metal – which can be deposited or removed – acts as a surfactant, lowering the surface tension between the metal and the surrounding fluid.

The researchers used a liquid metal alloy of gallium and indium. In base, the bare alloy has a high surface tension of about 500 millinewtons (mN)/metre, which causes the metal to bead up into a spherical blob.

‘But we discovered that applying a small, positive charge – less than one volt – causes an electrochemical reaction that creates an oxide layer on the surface of the metal, dramatically lowering the surface tension from 500mN/meter to around 2mN/meter,’ said Dr Michael Dickey, an associate professor of chemical and biomolecular engineering at NC State and senior author of a paper describing the work. ‘This change allows the liquid metal to spread out like a pancake, due to gravity.’

According to NC State, the researchers also showed that the change in surface tension is reversible. If researchers change the polarity of the charge from positive to negative, the oxide is eliminated and high surface tension is restored. The surface tension can be tuned between these two extremes by varying the voltage in small steps.

‘The resulting changes in surface tension are among the largest ever reported, which is remarkable considering it can be manipulated by less than one volt,’ Dickey said in a statement. ‘We can use this technique to control the movement of liquid metals, allowing us to change the shape of antennas and complete or break circuits. It could also be used in microfluidic channels, MEMS, or photonic and optical devices. Many materials form surface oxides, so the work could extend beyond the liquid metals studied here.’

Dickey’s lab had previously demonstrated a technique for 3D printing liquid metals, which used the oxide layer formed in air to help the liquid metal retain its shape, the opposite of what the oxide layer does to the alloy in a basic solution.

‘We think the oxide’s mechanical properties are different in a basic environment than they are in ambient air,’ said Dickey.

The paper, Giant and Switchable Surface Activity of Liquid Metal via Surface Oxidation, was published online in the Proceedings of the National Academy of Sciences September 16. Lead authors of the paper are Mohammad Rashed Khan and Collin Eaker, Ph.D. students at NC State. The paper was co-authored by Dr. Edmond Bowden, a professor of chemistry at NC State.